Process for precipitating polymer from aqueous emulsion



Patented Jan. 11, 1949 PROCESS FOR PRECIPITATING POLYMER FROM AQUEOUSEMULSION Louis Plambeck, Jr., Wilmington, DeL, assignor to E.,I. du Pontde Nemours 8: Company, Wilmington, Del., a corporation of Delaware NoDrawing.

Application February 19, 1946, Serial No. 648,815

6 Claims. (01. ace-29.6)

This invention relates to synthetic polymeric materials. Moreparticularly, it relates to a process for isolating synthetic polymericmaterials by coagulation of their aqueous dispersions.

This application is a continuation-in-part of my copending applicationSerial No. 518,411, filed January 15, 1944, now abandoned.

One of the preferred methods of preparing synthetic polymeric materialsconsists in carrying out the polymerization of the parent monomer inaqueous emulsion. This method, which is widely used in industrialpractice, has the advantage that the polymerization is in general rapidand readily controllable. The isolation of polymers from the resultingaqueous dispersions is usually carried out by freezing, by spray drying,by the addition of a water-soluble precipitant such as alcohol oracetone, or, preferably, by the addition of a water-soluble electrolyte.In large scale operations, the addition of an electrolyte is the mostwidely used isolation method. However, although this technique is verysatisfactory for unsaturated polymers such as the rubbery polymers ofconjugated 'dienes such as butadiene and chloroprene and for lowsoftening polymers such as polyvinyl acetate, it is not entirelysatisfactory for dispersions of high softening, waterinsensitive,essentially saturated polymers, 1. e. polymers free from aliphatic typecarbon-to-carbon unsaturation such as the polymers and copolymers ofvinyl chloride, methyl methacrylate, etc. With a polymer dispersion ofthis type, the addition of electrolytes results, in general, in aprecipitate which is so fine and slimy that it is difficult to wash itfree from impurities such as dispersing agents, catalyst residues, etc.Polymer precipitates of thisnature are objectionable in I industrialpractice becaus of their tendency to pass through the filters andeventually to clog them partially or completely.

An object of this invention is to provide a new process of coagulatingaqueous dispersions of high softening, essentially saturated polymers ofmonoethylenic monomers. Another object is to provide a polymercoagulating process whereby the polymer is obtained in a form which iseasily filter a.ble and washable. Other objects will appear hereinafter,

These objects are accomplished by the following invention, wherein asynthetic addition polymer, essentially free from non-aromaticcarbonto-carbon unsaturation and having av softening point above about60 C., of a monomer or monomers having but one acyclic carbon-to-carbonunsaturation, and that ethylenic, is precipitated from its aqueousdispersion by adding thereto a small amount of a water-insoluble,unpolymerizable, swelling agent for the polymer, which agent is liquidat the operating temperature and has a vapor pressure at the boilingpoint .of water (100 C.) of at least mm. (Hg), and then a water-solubleelectrolyte. The precipitated polymeric material is then isolated.

In the preferred practice of this invention, a

smallamount, e. g., 5% to 10%, based on the weight of the polymer, of awater-immiscible, unpolymerlzable, swelling liquid hydrocarbon boilingbelow 200 C., such as toluene or xylene,

is added to the aqueous emulsion of the polymer,

followed by the addition of the electrolyte and, preferably, by heatingto or near the boiling point of the mixture in order to complete thecoagulation and to remove all or most -of the swelling agent. Thepresence of the swelling agent has the unexpected and surprising effectof causing the polymer, upon subsequent addition of an electrolyte, toprecipitate in the form of coarse, easily fllterable'and washableparticles, rather than as the finely divided, difficult-tohandle slurrywhich results when the electrolyte is used alone.

The invention will be better understood by reference to the followingillustrative examples, in which parts are by weight.

Example I To 275 parts of a vinyl chloride/diethyl fumarate (86/14)copolymer emulsion containing 32% of copolymer having a softening pointof about C. and prepared as described below is added at room temperatureand with vigorous stirring 8.8 parts of toluene (about 10% based on thesolids content of the latex). After thorough mixing, there is added tothe suspension 15 parts of a 10% aqueous solution of aluminum sulfate.The mobile latex coagulatcs to a' soft, thick mass which thins out to aslurry on continued stirring. This slurry is then heated to to C. byinjection of steam while stirring, whereupon the polymer particlesbecome harder and denser, while most of the toluene is driven off. Whena test sample shows that the polymer settles rapidly and easily, heatingand stirring are discontinued and the precipitate is filtered and washedfree of electrolyte, dispersing agent, etc. by decantation andfiltering.

The copolymer of this example is prepared, as

I described in application Serial No. 427,921, filed January 23, 1942,by H. W. Arnold, now Patent No.

autoclave at 40 C. an aqueous emulsion, brought to a pH of 2.5, ofvinylchloride with about V5 of the total diethyl iumarate and adding theremainder of the diethyl fumarate during a period of 24 hours, afterwhich the polymerization is completed by further heating of one hour;There is used as the dispersing agent a commercial product containing-35% of an alkali metal salt of sulfonated paraffin oil and as thecatalyst a small amount of ammonium perdisulfate.

When the addition of toluene is omitted, other conditions being thesame, the polymer precipiproduct obtained in the presence of toluene.

Example [I A polyvinyl chloride dispersion containing 33% solids isprepared, as described in application Serial No. 427,922, filed byArnold, Brubaker and Dorough on January 23, 1942, now Patent No.2,404,781 issued July 30, 1946, by stirring for four hours at C. in anautoclave an emulsion (brought to pH 2.5) of vinyl chloride in watercontaining ammonium perdisulfate as the catalyst and the sodium salt ofsulfonated paraffin oil as the dispersing agent. The polyvinyl chlorideso obtained has a softening point of about 83 C. To 150 parts of thisdispersion is added 4.3 parts of toluene and 250 parts of water at roomtemperature with vigorous stirring. The

. .polymer is then precipitated by adding to'the dispersion five partsof 10% aqueous aluminum sulfate, after which steam is led into themixture, while stirring is continued, to bring the temperature to 85 to90 C., which completes the coagulation and drives off the toluene. Theresulting hard, rapidly settling polymer particles are washed threetimes with water by decantation, then washed on a filter with water andmethanol. Washing and filtering offer no difficulties whatsoever,whereas the same polymer, when coagulated under similar conditionsexcept that toluene is omitted, can be washed and filtered only throughlengthy and laborious manipulations.

Example III Monomeric methyl methacrylate is polymerized in aqueousemulsion at a temperature below 60 C. with 0.1% sodium bisulfitecatalyst and commercial sodium cetyl sulfate (3.3% active ingredientbased on the monomer) as dispersing agent to give a dispersioncontaining 30% to 33% by are easily washed to remove the soluble saltsand the impurities. The polymer is then air dried to remove water andany remaining toluene.

In the case of a rather high softening polymer like the high molecularweight polymethyl methacrylate of this example, it is advantageous touse more or the swelling agent than is needed with somewhat lowersoftening polymers, e. g., those of the preceding examples. If, insteadof 8.7 parts of toluene, there is used only 3.5 parts per 100 parts ofthe above described polymer emulsion (about 10% based on the solidscontent) the polymer is again precipitated in a satisfactory state ofdivision, but the particles are not quite as hard and tend to sticktogether, with the result that the rate of flow of the wash water on thefilter is somewhat decreased. If no toluene'is used, however, washingand filtering operations become a practical impossibility.

The process of this invention is applicable to the coagulation ofaqueous emulsions of any high softening, water-insensitive syntheticpolymer essentially free from ethylenic unsaturation, i. e., any.polymer of a monomer or monomers having but one non-aromaticcarbon-to-carbon unsaturation, and that ethylenic. Non-aromaticcarbon-to-carbon unsaturation is readily determined by thedecolorlzation of bromine solution without evolution of hydrogen.bromide. The term softening point is used here to denote the point ofsoftening under load, and is not to be confused with sticking point ormelting point. The softening point is determined, as described inBritish Patent 570,331. This is a test similar to A. S. T. M. D648-45T(see also Simonds and Ellis, Handbook of Plastics, page 98) but givessomewhat higher values. The procedure in this test is as follows:

A molded bar of the material, 2.5 x 0.5 x 0.05", is rigidly clamped atone end and loaded with. a 27.5 gramweight applied 2" from the point ofsupport and in the plane of the 0.5" dimension. The softening point isdefined as that temperature, plus or minus 1 C., at which the bardeforms 0.06" when it is heated in a liquid bath at the rate of 2 C. perminute.

The polymers to which the process of the present invention isparticularly applicable have a. softening point above about 0., sincelower softening polymers of monoethylenic compounds, as well asunsaturated polymers such as the diene polymers and copolymers, can bereadily coagulated from their aqueous emulsions by conventional methods.Furthermore, the process is es sentially of advantage with polymerswithout plasticizers and is of special benefit with polymers (includingco-polymers) of monoethylenic vinyl and vinylidene compounds, 1. e.,compounds having a terminal methylene (CH2) group attached to the restof the molecule through an ethylenic double bond. In addition to thepolymers of the examples, the process is applicable to many others,

including polystyrene, polyvinyl bromide, polyvinylidene chloride,copolymers of vinyl chloride and dialkyl maleates (or fumarates),copolymers of vinyl chloride and vinyl acetate, co-

' polymers of vinyl chloride and methyl acrylate,

copolymers of vinyl chloride and vinylidene chloride, etc. Aparticularly preferred copolymer is the vinyl chloride-diethyl fumarate(:5) polymer, which lends itself particularly well to the technique ofthisinvention.

Any unpolymerizable swelling agent for the polymer which is liquid atthe temperature of coagulation and substantially water-insoluble may beused, provided it is sufilciently volatile to permit easy removalthrough evaporation or heating, if desired. In practice, this means thatthe swelling agent should have a vapor pressure of at least 15 mm. atthe boiling point of water C.). The function of the swelling agent isbelieved to be that of a temporary .plasticlzer which improves thecharacter of the coagulum, yet is easily removed therefrom. It is forthis reason that other plasticizers should not :be present. Thus theswelling agents are Sharply distinguished in their action from theconventional plasticizers which are non-volatile substances designed toreagent and the electrolyte at room temperature,

main in the polymer permanently. Among the suitable swelling agents maybe mentioned benzene, toluene, o-, mand p-xylene, chlorobenzene,

nitrobenzene, anisol, trlchloroethylene, carbon For this purpose, aliquid is deemed to be a swelling agent when the finely divided polymer,upon immersion in the liquid (at or below the temperature of coagulationof the aqueous polymer suspension) increases in volume at least 5%within a short time, e. g., within less than 15 minutes. The preferredswelling agents, from the standpoint of satisfactory results with manypolymers and economy, are the aromatic hydrocarbons boiling below 2000., in particular, toluene and the commercial mixture of xylenes. Ifdesired, two or more volatile swelling agents may be used incombination. The swelling agent must be substantially.unpolymerizableunder the experimental conditions used, 1. e., up to temperatures of atleast 100 C. since a polymerizable swelling agent would act in efiectlike excess polymeriz able monomer, which usually has no beneficialinfluence onthe nature of the coagulate and causes undesirable results(e. g., crazing of films) if present in the final polymer. The swellingagent must be substantially insoluble in water at temperatures up to 100C. if satisfactory results are to be obtained. It must have an adequatevapor pressure at this point if it is to be conveniently and practicallyremoved by methods available to the industry, e. g. steam distillation.

With most polymeric materials it is unnecessary to use more than 3% to10% of the swelling agent, based on the weight of the polymer present inthe emulsion. However, with high softening polymers, it is desirable touse more of the swelling agent than with lower softening polymers, e.g., up to by weight of the polymer) and in extreme cases, even up to -Anexcess of swelling agent beyond the necessary quantity should be avoidedsince too much swelling agent may make the polymer particles coalesce ina gummy mass. For practical purposes, there should not be used .morethan of the swelling agent, the preferred range being'between about 3%and about 15%.

In the coagulation step, there may be employed any of the water-solubleelectrolytes well known in the art for that use. Two particularly goodelectrolytes are aluminum sulfate and calcium chloride. Othersatisfactory coagulants are sodium chloride, ammonium chloride,potassium bromide, magnesium sulfate, barium chloride, lead acetate,sodium sulfate, etc. Organic electrolytes such as sodiumtrichloroacetate may be used, but the inorganic salts are in generalcheaper and give better results. The preferred electrolytes are thestrong acid salts of polyvalent metals. The electrolyte may be added asa solid, or, preferably, as an aqueous solution. In general, it isdesirable to use from about 0.5% to about 30% of electrolyte, based onthe weight of the polymer.

The coagulation may be carried out at any desired temperature up to theboiling point of the. emulsion. It is often desirable to add theswelling cluded within the scope of the claims. 7

then to raise the temperature to 80% to 100% C. since a harder polymerprecipitate is often obtained by following this procedure. If theswelling agent is low boiling, the operation may be carried out in aclosed vessel.

In the preferred practice of the invention, the swelling agent is addedonly after polymerization of the monomer is complete or substantiallycomplete since if it is added during the polymerization, the rate of thelatter is, in general, decreased and, furthermore, there is the dangerthat part or all of the swelling agent may evaporate. This is especiallytrue if the latex is steamed afterpolymerization to remove anyunpolymerized monomer. It is also preferred to add the swelling agentbefore the electrolyte since if added after the electrolyte, it does notreadily come in contact with the particles of precipitatated polymer.

Preferably, the bulk of the swelling agent is removed during orimmediately after coagulation of the polymer by steaming, i. e.,bringing the aqueous reaction medium to an elevated temperature, e. g.,between 75 C. and its boiling point. If desired, removal of the swellingagent may also be effected by simply filtering off the polymer, washingit and drying it in air or at reduced pressure, if necessary with gentleheating, or both methods may be combined. It is in general harmless toleave minute amounts of the swelling agent adhering to the polymer sinceit will not be present in sufiicient amounts to cause permanentplasticization.

' The above description and examples are intended to be illustrativeonly. Any modification thereof or variation therefrom which conforms tothe spirit of the invention is intended to be in- What is claimed is:

1.,Process for obtaining, in form suitable for isolation by filtration,a macromolecular poTymer of monomer components consisting of vinylchloride and diethyl fumarate, which polymer is precipitated fromaqueous emulsion by water-soluble electrolytes in a finely divided anddifilcuitly file trable form, which comprises adding from 3 to 15%,based on the weight of the polymer, of toluene to a completelypolymerized aqueous emulsion of said polymer, coagulating the emulsionby the addition thereto of aluminum sulfate and thereafter removing thetoluene. I

2. Process for obtaining, in form suitable for isolation by filtration,a macromolecular saturated vinyl chloride polymer which, in the form ofa molded bar 2.5 by 0.5 by 0.05 inch when rigidly clamped at one end,loaded with a 27.5 gram weight applied two inches from the point ofsupport and in the plane of the 0.05 inch dimension, and heated in aliquid bath at the rate of two degrees centlgrade per minute does notdeform 0.06 inch below 60 C. and which polymer is precipitated fromaqueous emulsion by water-soluble electrolytes ina finely divided anddifiicultly filtrable form, which comprises adding from 3 to 15%, basedon the weight of the polymer, of toluene to a completely polymerizedaqueous emulsion of said polymer, coagulating the emulsion by theaddition thereto of a water-soluble salt of a polyvalent metal and aninorganic acid and thereafter removing the toluene.

3. In the isoiation by coagulation of aqueous emulsion and filtrationtherefrom of a saturated addition polymer which, in the form of a moldedbar 2.5 by 0.5 by 0.05 inch when rigidly clamped at one end, loaded witha 27.5 gram weight applied two inches irom the point of support and inthe plane or the 0.05 inch dimension, and heated in a liquid bath at therate or two degrees centigrade per minute does not deform 0.06 inch'below 60 C. and of a monomer composition in which each monomer has butone aliphatic carbon-to-carbon unsaturation and that is ethylenic, whichpolymer is precipitated from aqueous emulsion by water-solubleelectrolytes in a finelydivided and difilcultly flltrable form, theimprovement which comprises adding to the aqueous emulsion of saidpolymer, prior to the coagulation by a water-soluble electrolyte, awater-insoluble, unpolymerizable, aromatic liquid hydrocarbon swellingagent for said polymer, which swelling agent .boils below 200 C., inamount from 3 to 30%, based on the weight of the polymer, but less thanthe amount sufficient to cause the polymer to coalesce to a gummy massand, after coagulation,

evaporatively removing the liquid hydrocarbon.

4. In the isolation by coagulation of aqueous emulsion and filtrationtherefrom oi a saturated addition polymer of a monomer composition inwhich each monomer has but one aliphatic carbon-to-carbon unsaturationand that is ethylenic, which polymer, in the form of a molded bar 2.5

by 0.5 by 0.05 inch when rigidly clamped at one' end, loaded with a 27.5g am weight applied two inches from the point of support and in theplane of the 0.05 inch dimension, andheated in a liquid bath at the rateof two degrees centigrade per minute does not deform 0.06 inch below 60C. and which polymer is precipitated from aqueous emulsion bywater-soluble electrolytes in a finely-divided and difilcultly filtrableform. the improvement which comprises adding to the aqueous emulsion ofsaid polymer, prior to the coagulation by a water-soluble electrolyte, awater-insoluble, unpolymerizable liquid organic swelling agent for saidpolymer which swelling agent has a vapor pressure at 100 C. of at least15 mm. in amount from 3 to 30%, based on the weight of the polymer, butless than the amount sufilcient to cause the polymer to coalesce to agummy mass and, after coagulation, evaporatively removing the swellingagent.

5. Process for obtaining, in fcrmsuitable for isolation by filtration. asaturated addition polymer of a monomer composition wherein each monomerhas but one aliphatic carbon-to-carbon unsaturation and that isethylenic, which polymer, in the form of a molded bar 2.5 by 0.5 by 0.05inch when rigidly clamped at one end, loaded with a 27.5 gram weightapplied two inches from the point of support and in the plane of the0.05 inch dimension, and heated in a liquid bath at the rate of twodegrees centigrade per minute does not deform 0.06 inch below to c. andwhich poylmer is precipitated from its aque ous emulsion in finelydivided and difllculty filtrable form, which comprises adding to acompletely polymerized aqueous emulsion of said polymer awater-insoluble, unpolymerizable, liquid organic swelling agent for saidpolymer, which swelling agent has a vapor pressure at 100 C. of at leastmm., in amount from 3 to based on the weight oi the polymer but lessthan that sunlcient to cause coalescence of the polymer particles to agummy mass, coagulating-the aqueous polymer emulsion by the additionthereto of a water-soluble electrolyte and thereafter evaporativelyremoving the swelling agent. I

8. Process for obtainim, in form suitable for filtration, a polymer of amonomer composition in which each monomer has a CH2=C= group and thedouble bond on the terminal CH: is the only aliphatic carbon-to-carbonunsaturation,

Y which polymer in the form of a molded bar 2.5

by 0.5 by 0.05 inch when rigidly clamped at one end, loaded with a 27.5gram weight applied two inches from the point of support and in theplane of the 0.05 inch dimension, and heated in a liquid bath at therate of two degrees centigrade per minute does not deform 0.06 inchbelow C. and which polymer is coagulated from aqueous emulsion bywater-insoluble electrolytes in a finely-divided and difilcultlyflltrable form which comprises adding to the aqueous emulsion of saidpolymer a water-insoluble unpolymerizable, liquid organic swelling agentfor said polymer, which swelling agent has a vapor pressure at C. of atleast 15 mm., in amount from 3 to 30% based on the weight of the polymerbut but less than that suificient to cause coalescence of the polymerparticles to a gummy mass, coagulating the aqueous polymer emulsion bythe addition thereto or a water-soluble electrolyte, and thereafterevaporatively removing the swelling agent.

. LOUIS PECK, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,193,613 Alexander Mar. 12, 19402,337,681 Pollack Dec. 28, 1943 FOREIGN PATENTS Number Country Date691,382 Germany May 24, 1940 Certificate of Correction Patent No.2,458,636. January 11, 1949.

LOUIS PLAMBECK, JR.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows:

Column 6, line 2, for 80% to 100% 0. read 80 to 100 0.;

and that the said Letters Patent should be read with this correctiontherein that the same may conform to the record of the case in thePatent Ofiice.

Signed and sealed this 24th day of May, A. D. 1949.

THOMAS F. MURPHY,

Assistant Uommiasz'oner of Patents.

